A heat shrinkable, thermoplastic, multi-layer packaging film which has enjoyed considerable commercial success is described in U.S. Pat. No. 3,741,253 which issued on June 26, 1973 to H. J. Brax et al. A film disclosed in the Brax et al patent may be produced by extruding a first or substrate layer of ethylene-vinyl acetate copolymer as an unstretched tube and then flattening the tube and moving it in a series of passes through an electron beam to cross-link the molecules of the copolymer. Next, the film is inflated but not substantially stretched and passed through an annular coating die where it receives a second layer which is a copolymer of vinylidene chloride. The two layer tube then passes through another annular die in which it receives a coating of ethylene-vinyl acetate copolymer. The tube now has a wall of three polymeric layers and is cooled, collapsed, and fed through a hot water bath where the tube is sufficiently softened so that it can be inflated into a bubble which is trapped between two spaced apart pairs of pinch rollers. The inflation causes the tubing to be stretched into oriented film as the bubble cools quickly as it leaves the hot water bath. After the bubble is collapsed the film is wound up as flattened, seamless, tubular film to be later used to make bags, e.g. end-seal bags are typically made by transversely heat sealing across the width of the tubing followed by severing the tubing. Thus, the transverse seal will form the bottom of a bag.
In making bags as described above in a rapid commercial operation by pressing the flattened sheet walls together with heated seal bars, the dwell time of the seal bars should be as short as possible yet sufficiently long to fuse and bond the inside surface layer of the tube to itself in a smooth, continuous, strong seal. The importance of a strong reliable seal is readily understood when the use and function of the bag is described. This type of bag is primarily used to package a large variety of fresh and processed meats, cheese, and fish by placing the meat in the bag, evacuating the bag, gathering and applying a metal clip through the mouth of the bag to hermetically seal it, and then immersing the bag in a hot water bath at approximately the same temperature at which the film was stretch-oriented. This temperature will typically run from 160.degree. to 200.degree. F. One problem which has been encountered is the failure of the bag seals at the bottom of the bag as the bag is shrunk around the meat and the shrink forces tend to pull the seal apart. It was observed that an inflated bag immersed in hot water would have a more reliable seal than a bag filled with meat even though the unfilled bag had been inflated to a pressure which would duplicate the same forces on the seal. It appears that animal fat, particularly fat from processed meats, dissolves in the ethylene-vinyl acetate copolymer layer in the seal area, tends to act like a plasticizer, and softens and weakens the seal. One way to solve this problem was to increase the dwell time of the seal bars to make a stronger seal and also to use wider seals. However, this would reduce the production speed of bags and waste bag material. Accordingly, it is one object of the present invention to provide an improved film from which bags can be made and which will not rupture or pull apart at elevated temperatures.
Another problem encountered in handling and processing bags of a thermoplastic material having a low melting point is that of tackiness, i.e., the tendency of the bag material to stick to itself. Thus, it is another object of the present invention to make a multi-layer laminate where the layer having the greatest degree of tackiness is an interior layer rather than a surface or skin layer.
Still another object of the present invention is to improve seal reliability when using low melting temperature resins in a multi-layer film.
In the film of the Brax et al patent the vinylidene chloride comprises the middle layer. This layer is important because the vinylidene chloride copolymer has very low gas transmission whereas, by comparison, the ethylene-vinyl acetate copolymers which provide the shrink and sealing layers do not. Thus, another object of the present invention is to provide shrink and gas transmission properties at least approximately equal to the prior art film and at the same time have improved sealing and processing characteristics.
The foregoing objects are achieved by the present invention which is summarized in the paragraphs below.